The great earthquake of 2047 that sank the Los Angeles area beneath some two hundred metres of water, later creating one of Earth's largest coral reefs.
The Hermosa Earthquake is described in the Voyager two-part episode Future's End. When Voyager is thrown back to Earth in 1996 and enters orbit over southern California, Janeway and her officers recall that the region did not survive into their own time in the same form.
In 2047 the quake caused the Los Angeles area to sink under about two hundred metres of water. Over the following centuries the drowned region became one of the largest coral reefs on the planet and a home to thousands of marine species.
The disaster falls within the years of World War III in Earth's history, although the episode does not connect the two. Tom Paris, an enthusiast for the 20th century, is the crew's guide to the city as it was before the quake.